Recently, according to downsizing and high density of electronic devices, multilayer of a circuit board has been strongly demanded in a field of not only industrial use but also public use. A connecting method, by which circuit patterns of a plurality of layers are connected to each other through inner via-holes, and a structure having high reliability are necessary for the circuit board mentioned above. An example of the method for manufacturing the circuit board structured by using the inner via-hole connection with conductive paste is disclosed in Unexamined Japanese Patent Publication No. H6-268345 or No. H7-106760.
The conventional method for manufacturing a double-sided circuit board is described hereinafter. FIGS. 10A through 10F are sectional views showing steps of the conventional method for manufacturing the circuit board. FIG. 11 is a perspective view showing a conventional frame of a printing plate fixing a mask having an opening. FIG. 12 is a sectional view showing the conventional frame of the printing plate fixing the mask having the opening. FIGS. 13A through 13G are sectional views showing steps of filling paste by using a squeezing method. FIG. 14 is a partially sectional view of the conventional circuit board while the paste is filled. Prepreg sheet 21 shown in FIG. 10 has a size of 300 mm×500 mm and a thickness of approximately 150 μm. For example, a substrate formed of a composite, which is wholly aromatic polyamide fiber of nonwoven fabric impregnated with thermosetting epoxy resin, is used as prepreg sheet 21. Plastic film having a thickness of approximately 20 cm and a width of approximately 300 μm, for example polyethylene terephthalate (hereinafter referred to as “PET”), is used as mask films 22a and 22b. Si base mold-releasing layer not thicker than 0.01 μm is formed on a surface, which is bonded to prepreg sheet 21, of the mask film. A laminate apparatus is used for attaching prepreg sheet 21 to mask films 22a-22b. Mask films 22a and 22b are continuously bonded to prepreg sheet 21 by melting resin ingredient of prepreg sheet 21. Through-hole 23 is filled with conductive paste 24, thereby being electrically connected with metal foils 25a-25b which are copper or the like of a 35 μm thickness and attached to both sides of prepreg sheet 21.
The method for manufacturing the circuit board is described hereinafter. First, as shown in FIG. 10B, through-holes 23 are formed by using a laser processing method or the like at certain points of prepreg sheet 21 (FIG. 10A) whose both sides are bonded to mask films 22a and 22b. 
Next, as shown in FIG. 10C, through-hole 23 is filled with conductive paste 24. A method for filling conductive paste 24 is described hereinafter. Prepreg sheet 21 including through-holes 23 is disposed on a stage of a general printing machine (not shown). By reciprocating two squeegees such as urethane rubber alternately, conductive paste 24 is filled directly from above mask film 22a. At this time, mask films 22a and 22b act as printing masks and prevention of pollution on surfaces of prepreg sheet 21.
The method for filling conductive paste 24 is further discussed hereinafter with reference to FIGS. 11, 12, and 13A-13G. A squeezing method is used for filling conductive paste 24. Because prepreg sheet 21 has particular mask films 22a and 22b, mask 32 made of stainless having a thickness of approximately 3 mm and forming opening 33 of 250 mm×450 mm larger than an effective area, in which paste is filled, of prepreg sheet 21 is fixed at frame 31 of printing plate 30 for filling as shown in FIGS. 11 and 12. First, as shown in FIG. 13A, mask 32 is set to prepreg sheet 21 disposed on stage 35 of the printing machine (not shown). The mask films are bonded to both sides of prepreg sheet 21, and through-hole 23 is formed.
Next, forward-squeegee 36a and backward-squeegee 36b capable of moving perpendicularly and horizontally, and pressing, have been disposed at an upper side of the printing machine. Only forward squeegee 36a descends to a certain position on mask 32 and pressed, so that conductive paste 24 is rolled and advanced.
Then, as shown in FIG. 13B, forward-squeegee 36a passes through slope part 34 of mask 32, and reaches an upper surface of prepreg sheet 21. Forward-squeegee 36a and backward-squeegee 36b have functions capable of moving perpendicularly and freely with respect to a position with keeping pressure.
Next, as shown in FIG. 13C, forward-squeegee 36a passes through the upper surface of prepreg sheet 21 and a slope part of mask 32, and stops at a predetermined position. After that, forward-squeegee 36a ascends, so that conductive paste 24 is dropped naturally.
Then, as shown in FIG. 13D, only backward-squeegee 36b descends to a certain position on mask 32. After that, as shown in FIGS. 13E through 13G, backward-squeegee 36b passes through mask 32 and the upper surface of prepreg sheet 21, so that a process of filling conductive paste 24 into through-hole 23 is finished.
Further, as shown in FIG. 10D, mask films 22a and 22b are peeled from the both sides of prepreg sheet 21. Next, as shown in FIG. 10E, metal foils 25a and 25b such as copper are piled at the both sides of prepreg sheet 21. By heating and pressing with a heat press in this state, as shown in FIG. 10F, prepreg sheet 21 is compressed in thickness (t2=approximately 100 μm) and prepreg sheet 21 and metal foils 25a-25b are bonded to each other. As discussed above, metal foils 25a and 25b at both sides are electrically coupled with each other by using conductive paste 24 filled in through-holes 23 formed at a certain position. Furthermore, metal foils 25a and 25b at both sides are etched selectively, so that circuit patterns (not shown) are formed and the double-sided circuit board can be obtained.
Next, problems of the conventional paste-filling method mentioned above are described hereinafter with reference to FIG. 14. In starting of printing, in a case of high viscosity of conductive paste 24, when squeegee 36b descends slope part 34 of mask 32, conductive paste 24 is pressed to a surface of prepreg sheet 21, so that resin ingredient is pushed out of paste around an edge of squeegee 36b. Therefore, conductive paste 24 having high viscosity adheres to the mask film by whole edge of squeegee 36b. When this adhered conductive paste 24 passes through through-hole 23 formed at prepreg sheet 21, solid paste tends to remain on through-hole 23 which is the first through-hole in a moving direction of the squeegee. Particularly it is revealed at whole width, where paste is filled, of squeegee 36b. Particularly, as shown in FIG. 15, in a case where the mask film has a thickness of 20 μm and a diameter of through-hole 23 is not larger than 150 μm, when mask films 22a and 22b are peeled, a part of conductive paste 24 is removed to a side of mask films 22a. As a result, conductive paste 24 in through-hole 23 becomes insufficient, so that there is a possibility that quality of connection is adversely affected.